1 Field of the Invention
The invention relates to an electronic current-limiting device having a superconducting switching element which is arranged in a cryostat.
2 Description of the Related Art
A current-limiting device of the type mentioned in the introduction is known, for example, from DE 102 30 084 A1. This current-limiting device is operated in an electrical network. The current is carried by the superconducting switching element, which has a negligible small electrical resistance at the operating temperature which is maintained by continuous cooling. If a network fault (for example a short) occurs in the network, then the unacceptably high short-circuit current results in the superconducting switching element having a response which is referred to as quenching. The high current flow produces induction in the super conductor, which first of all slightly increases its electrical resistance. This changes the superconductor to the so-called flux-flow range, in which the superconducting material is in the Shubnikov phase. This response is described, for example, in W. Buckel, Supraleitung: Grandlagen und Anwendungen [Superconduction: Principles and Applications], Weinheim 1990. According to this document, normally conductive areas and superconducting areas are formed at the same time in the superconducting material in the Shubnikov phase and are formed by migrating normally conductive flux tubes in the said conductor. The increase in the resistance of the superconducting switching element associated with this leads to heating of the superconductor. This can also be accelerated if, as is generally normal, a communication conductor is applied as a layer on the superconductor and carries a portion of the short-circuit current when the superconductor enters the flux-flow range, producing additional heat because its resistance is considerably greater. The heating of the switching element leads in a very short time to a critical temperature being exceeded, above which the superconductor is normally conductive, that is to say it leaves the flux-flow range as a function of the temperature, and limits the short-circuit current to a short-circuit residual current owing to its suddenly rising resistance, which is now purely resistive.
In order to prevent damage to the superconducting switching element when the current is being limited, the switching element must have the load removed from it again at the latest 50 ms after response, and this is done for example by means of an electrical bypass element, whose resistance provides the current limiting. The superconducting switching element is disconnected from the network and cannot be operated again until it has cooled down to the original operating temperature.
DE 102 30 618 A1 describes a design for a superconducting current limiter which is achieved by the superconductor being in the form of a strip. The superconductor in the form of a strip is arranged in a spiral shape in the coolant, in such a manner that the coolant can reach the surface of the superconductor between the spiral turns. This makes it possible to shorten the cooling-down time after current limiting, so that the current-limiting device can be used relatively quickly again after the current has been limited. However, the described principle of operation means that cooling is required before the current limiter is used again.